There are three main technologies that are used for the production of syngas from methane and they are steam reforming, autothermal reforming and partial oxidation (catalytic and non-catalytic). The most commonly used are autothermal and steam reforming or a combination of the two. Both these technologies require a large proportion of steam to be included with the methane feed to prevent coke formation and reforming catalyst deactivation. In order to achieve high energy efficiency the large amount of sensible and latent heat contained within the steam must be recovered and recycled to the process.
Non-catalytic partial oxidation does not require the high levels of steam but the very high process temperatures (>1200 deg C.) create energy efficiency challenges of their own.
One more recent, non-commercial technology is the catalytic partial oxidation of methane using rhodium catalysts. Rhodium has been found to be highly selective in the oxidation with minimal coke formation allowing the partial oxidation process to be run at much lower temperatures. The process doesn't require steam to operate, although small quantities (10 vol % of the methane feed) are frequently described as a means of increasing the hydrogen to carbon monoxide ratio in the resultant syngas.
The simplicity of the system, with little or no steam, a lower temperature of operation and a highly active catalyst promises a compact and efficient process that is capable of operating efficiently without extensive steam recycles. However, the processes described in the literature prior to U.S. Pat. No. 7,641,888 Gobina, utilise a pre-mixed feed well within the explosive limits of the gases to produce a selective reaction. U.S. Pat. No. 7,641,888 is incorporated herein by reference. This presents significant safety problems particularly in operation and preheating of the respective feeds. The safety of the reaction relies on the gas velocities being maintained at a sufficiently high speed that flash back to the inlet point does not occur.
With the invention of a two chamber reactor separated by a porous, catalytic, membrane with mixing and reaction taking place simultaneously within the reactor the safety of the system was greatly improved.
However, there is another problem that is found within a fixed bed partial oxidation reactor that is described in the literature but not referred to in U.S. Pat. No. 7,641,888. That is the problem of catalyst overheating. It has since been found that a similar problem can also occur within the two chamber porous membrane reactor described. The steps to mitigate this problem within a multitubular reactor are the subject of this patent.
The partial oxidation of methane is a very rapid reaction that takes place at temperatures in excess of 600 deg C. Typically, when performed using a fixed bed of catalyst with a pre-mixed feed comprising methane and oxygen (gas molar ratio of 2:1) the feed is preheated to at least 400 deg C. such that good selectivity to carbon monoxide is achieved. The temperature of the gases passing over the catalyst rapidly rises and under adiabatic conditions (no heat loss) the product gases leaving the reactor can be in excess of 900 deg C. It is also beneficial if the reaction can be performed at elevated pressure since most of the processes that utilise syngas to form another chemical do so at raised pressure and the costs of compressing the component feed streams (comprising methane and oxygen) is less than compressing the resultant syngas. This is principally as a result of the increase in gas volumes that accompany the reaction. The partial oxidation of methane as described in U.S. Pat. No. 7,641,888 is found to have similar characteristics in that it is most beneficially carried out at elevated temperature and pressure.
The drawback of performing the partial oxidation reaction in a simple adiabatic reactor is that there is no control on the temperature of the fluids within the reactor and so there is less flexibility to operate the reactor at a temperature that is most beneficial for maintaining a long catalyst life. With a typical long contact time reactor this is a straightforward problem to solve by an engineer skilled in the art. Placing the catalyst pellets within the tube of a shell and tube type reactor or using a tube cooled reactors are both possibilities. A further possibility useful in lower temperature reactor is to operate the reaction in the liquid phase where the heat capacity of the liquid is able to absorb the heat of reaction.
Where a reaction such as the partial oxidation of methane requires a short contact time at high temperature, typically using a very shallow catalyst bed of pellets or gauze, then the removal of heat is more problematic. Unusual solutions can be found such as in the silver catalysed methanol to formaldehyde reactor where good thermal contact between enlarged, sintered catalyst pellets allows conduction of the heat of reaction to the front of the bed, which then acts as a feed pre-heater producing an essentially isothermal catalyst bed within an adiabatic reactor.
In the operation of a fixed bed catalyst with pre-mixed feed for the oxidation of methane to syngas there are safety issues that are associated with operating in an explosive regime. These problems are exacerbated if heat exchange function is required within the reactor. Some have sought to counteract this by stage wise addition of oxygen to the feed methane requiring a complex series of fixed beds and gas distributors (as described in Conoco U.S. Pat. No. 7,261,751 which is incorporated herein by reference) and this allows for removal of some of the reaction heat between catalyst beds as the material contains little or no oxygen as it passes through the heat exchanger. However, this is a complex and expensive solution.
A problem, found with rhodium partial oxidation catalysts in a fixed bed arrangement, is that despite the high selectivity that is achievable with this form of catalyst very high catalyst surface temperatures can form that far exceed the adiabatic reaction temperature.
One option to reduce the catalyst surface temperatures within the reactor is to operate the catalyst with a turbulent gas in contact with the catalyst. This is the subject of the present invention.
The slow partial oxidation of methane with oxygen is known to be a strongly exothermic reaction followed by an endothermic reaction. After this initial discovery it was discovered that the reaction would still take place at very much higher gas hourly space velocities (GHSV). The fast partial oxidation of methane with oxygen using a fixed bed of rhodium on alumina was thought to be effectively isothermal, although there is still some debate on this. Published work has shown that the reaction pathway still involves high heat release in the initial part of the catalyst bed and endothermic reactions later (e.g. Basini, Aasberg-Petersen, Guarinoni, Ostberg, Catalysis Today 64 (2001), 9-20, which is incorporated herein by reference). Some have attributed this rise in surface temperature to the superadiabatic effect that is related to the higher diffusion rates of H2 and H in combustion processes, others have suggested it is a consequence of competing kinetics. However, a satisfactory way of managing the heat profile of the catalyst bed has not been found.
We also refer to WO 2004/098750, which is incorporated herein by reference, which relates to a membrane and a method of preparing the membrane, said membrane being used in a process to produce hydrogen gas via a partial oxidation of methane.
It is an object of at least one aspect of the present invention to obviate or mitigate at least one or more of the aforementioned problems.
It is a further object of at least one aspect of the present invention to provide an improved process and apparatus for partial oxidation of methane.
It is a further object of at least one aspect of the present invention to provide an improved process and apparatus for partial oxidation of methane which enables catalyst in the reaction zone to be cooled and thereby overcome the problem of a reaction catalyst overheating.
It is a yet further object of at least one aspect of the present invention to provide an improved process and apparatus for partial oxidation of methane which allows higher temperatures to be used which increases the thermal efficiency of the process and which also allows higher pressures of operation.